If there is a hot day forecast, then pre-cool your house the night before. This is especially effective if your house has a high thermal mass such as solid brick. This will allow your air conditioner to work at its optimum during the cool evening, saving power and wear and tear on the unit. It also allows the unit to keep on top of the heat load as it begins to warm up during the heat of the day.

Running costs

Operating costs of electrical appliances such as fridges, heaters and air conditioners
Running costs and tips for heating and cooling.

Power kilowatts and Refrigeration kilowatts
Electrical input wattage or Hp, and refrigeration wattage are two different things.

A unit drawing 746W (1Hp) of electricity may move 2 to 3.5 times that in cooling or heating wattage. Only the latter refrigeration kilowatt (kW) rating will give an accurate representation of unit size.
Input power conversions.
To convert from Hp to kW multiply by 0.746
To convert from kW to Hp multiply by 1.34

Systems are judged on their efficiency by their Coefficient Of Performance (COP) in the case of heating, or the Energy Efficiency Ratio (EER) for cooling. The EER and COP are calculated by dividing the capacity output divided by the electrical input. More info on EER and COP and how they relate to star ratings can be found here. An interactive side-to-side comparison of air conditioners, showing energy efficiency ratings, power consumption and estimated running costs can be found here.
To work out how much your unit will cost to run multiply the kW input figure (electrical) by how much you pay per kilowatt hour for your electricity.
ie. 13.4c/Kwh x 1.28kw = 17.1 cents per hour to run.

Note: It is worth doing your own calculations before making a purchase. Some of the star ratings on the energyrating site are derived from incorrect manufacturers EER/COP figures and can give a false efficiency rating.

Single phase (240v) systems are available in ducted splits up to 18kW and non ducted splits up to ~14.5kW. Systems larger than this would require three phase power (415v). If you don't have it, you'd need to run new mains, replace the meter and rewire the main switchboard. This would be in the vicinity of $1500–$2000 as a rough guide. More if existing boards etc. need to be replaced.

Air conditioning sizing calculators
To get an approximation of the kWs required to cool an area try hereor here and put in the relevant details.
An evaporative cooling calculator is available here.

Refrigerant types R22, R410a, R407c, R417a
Older hydrochlorofluorocarbon (HCFC) based refrigerants are in the process of being phased out due to their ozone depleting potential (ODP). That means the current air conditioning refrigerant R22 has a phased reduction until 2015. From 2015 to 2030 a small amount of current levels is allowed to service existing equipment and none by 2030. Alternative refrigerants (drop ins) will most likely be available for older equipment so you will still be able to repair them using these.

The main replacement that is being introduced is hydrofluorocarbon (HFC) R410a which has an ODP of zero. Unfortunately it has a higher direct greenhouse warming potential (GWP) than R22 but indirectly it’s less damaging. R22 manufacturing by-products caused a far higher total GWP.
R407c is another of the R22 replacements that requires polyol ester oil.
R417a is a drop in replacement for R22 systems that requires no changes and uses the existing mineral oil. Will become the simplest replacement as R22 is phased out.

If you are considering purchasing a new air conditioner, it’s worth shopping for an R410a or R407c unit. They’re marginally more efficient, better for the environment and will be more future proof in terms of servicing Another benefit is reduced unit size relative to an R22 unit. Systems using R410A refrigerant run at a pressure of approximately 1.6 times that of similar systems using R22 so it can’t be used in existing systems due to different components such as compressors and the pipe wall thickness needing to be higher on R410a.

Legionella risks
Legionaires disease has been overblown by the media and it is not something you can contract from domestic air conditioning. The majority of cases are caused by incorrect cooling tower maintenance. Cooling towers are primarily used in commercial air conditioning to cool water cooled condensors and industry to cool recirculated water. They provide an ideal place for legionella bacteria to grow (temps between 28 to 40°C) and the operation of the towers causes drift which aerosols the bacteria allowing people to breathe it in. There’s probably more danger handling potting mix than from a cooling tower because by law all towers have to be maintained and treated with biocide. Evaporative coolers have been suggested but not verified as potential disease transmitters. Modern evaporative coolers also have dump valves which further lessen the risk.

The disease does not affect everyone who comes into contact with the bacteria. The groups most at risk are:
• People over 50 years of age (predominantly males)
• Heavy smokers
• Heavy drinkers
• Diabetics
• People with chronic lung disease
• People with impaired immune systems (ie body defence mechanisms)

Brands
The old adage ‘You get what you pay for’ exists for a reason. The dearer units such Daikin are generally (not always) more efficient and or quieter. They also go together better and give you a better finished product. This translates to easier servicing should the need arise which in my opinion makes the additional purchase costs worth considering. However, some of the cheaper brands provide excellent value for money and offer very good performance. Final decisions should be based on specific unit performance and value for money. There is very little difference between the mainstream and top units sold these days. It's only in the very cheap units that quality begins to suffer.

Source: http://whirlpool.net.au